He was born in Iran to Iraqi-Kurd parents who fled their home country in fear of persecution. He was at the top of the list when the under-15s basketball team at his high school was named just six months ago. He was into reality TV and soccer.

Jabar was known for having a troubled, but not troublesome, demeanour. Others at Arthur Phillip High School cannot remember him ever swearing. He was picked on, but was not one for violent retaliation. Of all the words used to describe him by those who have met him, the most common is “quiet”.

But sometime between his entering high school and his arrival outside NSW police headquarters at 4.30pm on Friday, Jabar embraced terrorism and killed Curtis Cheng

Authorities are still trying to piece together who caused that change in him, and why he decided to kill a NSW police employee who was unlucky enough to have walked out of that Charles Street building at the time.

All of Australia’s terrorist attacks and plots since September last year have been linked to Islamic State — currently fighting Kurdish forces in Syria and Iraq. It is also a solely Sunni jihadist group. But for nearly all of his life, Jabar counted Shia Muslims and even non-Muslims among his small number of friends. While watchingThe Voice on TV, he backed the team coached by Ricky Martin — an openly gay pop star. And he played team sports, which are frequently promoted by governments as a way to counter “violent extremism”.

“With a kid as young as 15, coming from the background that he came from where there’s no warning signs, I think we’ve got to assume that there are other people who came into his life and there was a degree of deliberate manipulation,” Deakin University terrorism expert Greg Barton said.

In that sense, Professor Barton said, it was unlikely Jabar was a “lone-wolf” — in terms of being an extremely isolated loner who organically self-radicalises.

Those who have known him, on the basketball court, in the playground and in the classroom, firmly agree. “His mind must have been washed by the fake scholars,” one upset boy said. Another fellow student painted a picture of Jabar as a sad loner who would play handball by himself and would rather stay at school until 4.30pm than head home on the final bell.

“He always seemed really cautious. He always looked upset,” the student said. “He was picked on a lot by the boys, he was an easy target. He only had about one or two friends.” Sport was one passion Jabar was known for. The other was religion. “He was so religious and quiet,” another schoolmate said. At the Parramatta Mosque — the last place he went before heading to NSW police — he would always try to be in the front row for prayers. He was there for nearly all of the daily prayers, even if it meant skipping school.

Often, he went with a group of boys of around the same age.

Even as a 10-year-old, Farhad appeared different to his peers. He created a Facebook page but most of the “friends” he added appear to be much older, and they would share photos of parties and girls and gang-style tattoos.

They may have been friends of Farhad’s older siblings, whose backgrounds are being examined by authorities. An older brother tipped off police about Jabar’s identity soon after Mr Cheng was gunned down on Friday.

Jabar’s older sister left the country last week, and authorities suspect she is destined for Syria. Like Jabar, she was not on the radar of police.